Within the modern network space, the Synochronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) protocol is becoming increasingly popular as a mechanism for data transport. In this respect, SDH is the European equivalent of the SONET transmission standard. Accordingly, all references in this application to SONET should be understood to also refer to SDH.
A significant amount of SONET/SDH infrastructure has been installed, particularly within the network core. This SONET infrastructure is used to transport asynchronous subscriber signal traffic having differing formats, such as Asynchronous Transfer Mode (ATM), Internet Protocol (IP), etc. In order to facilitate this functionality, various known methods are provided for mapping the asynchronous subscriber traffic into Synchronous Transfer Signal (STS/STM) frames for transport across the SONET infrastructure, and then extracting the subscriber traffic out of the STS to recover the original subscriber signal format.
FIG. 1a is a block diagram schematically illustrating principal operations of a conventional transmitting node 2 of an optical communications system. As shown in FIG. 1a, asynchronous subscriber traffic within multiple tributaries 4 is received by the node 2 and buffered in an elastic store 6. The traffic may comprise any arbitrary mix of signals, including DS-1, DS-3 and E1 traffic. Traffic within each tributary 4 is normally buffered in a respective First-In-First-Out (FIFO) buffer 8. The timing of this buffering operation is controlled by a data clock signal 10 having a frequency f1 generated by a tributary clock recovery circuit 12. A synchronizing framer (or “mapping unit”) 14 reads data from each FIFO 8, and maps the read data into corresponding tributaries of a number of SONET Synchronous Payload Envelopes (SPEs) 16, using a known format such as those defined in the SONET standard. Each SPE 16 is then passed to a channel transmitter (Tx) 18, which inserts the SPEs into an STS frame, and then modulates the STS frame onto an optical channel carrier 20 for transmission through the optical network. A Tx local clock 22, which is synchronous with a SONET Primary Reference 24, generates a respective TX local clock signal 26 having a frequency f2, which is used to control operation of the synchronizing framer 14 and channel Tx 18.
As is known in the art, the number and size of the SPEs 16 are selected based on the channel line rate. For example, for a channel line rate of 10 Gb/s, the synchronizing framer 14 may map subscriber traffic into a set of four STS-48 envelopes. Other combinations may equally be used, such as, for example, eight STS-12 envelopes.
Normally, a respective buffer fill signal 28 is generated for each tributary FIFO 8, and used to control the insertion of stuffing bits into the corresponding SPE tributary.
As shown in FIG. 1b, at the terminating node 30, the incoming STS 20 is decoded by a channel receiver (Rx) 32 and processed by a pointer processor 34 to demap each SPE tributary from the STS 20. Thus, stuffing bits are stripped out of each tributary, and the remaining subscriber data stored in a respective tributary FIFO 36 of an elastic store 38. An Rx local clock signal 40, having a frequency f3 which is preferably referenced to the SONET Primary Reference 24, is supplied to a desynchronizer Phase locked Loop (PLL) 42. A buffer fill signal 44 generated by the tributary FIFO 36 is used to steer the Phase locked Loop (PLL) 42, so that the PLL output constitutes a recovered data clock signal 46 having a frequency f4 which approximates the data rate of the subscriber traffic. As a result, by reading data from the tributary FIFO 36 at a timing of the recovered data clock 46, a desynchronizer framer 48 can generate a recovered subscriber signal 50 in which the original timing is closely approximated.
For cases in which the channel line rate is equal to or greater than the subscriber data rate (i.e. for f1≦f2), the introduction of idle packets to replace “missing” subscriber traffic enables the synchronizing and desynchronizing framers 14 and 48 to compensate any differences between the tributary data rate and the channel rate. However, this mapping technique suffers a limitation in that the fill signal 44 of the Rx tributary FIFO buffer 3b tends to vary in a step-wise manner as idle packets are inserted and striped from SPE tributaries. This causes timing jitter in the recovered subscriber signal 50.
In most situations, the amount of timing jitter introduced by mapping and demapping asynchronous client signal traffic to and from STS frames does not create any difficulties. However, if the timing of the subscriber signal is critical, such as an HDTV signal or a subscriber-originated SONET signal (e.g. for SONET over SONET applications) the introduced timing jitter can noticeably degrade the quality of the subscriber's signal. Accordingly, there is interest in methods that enable subscriber traffic to be transparently mapped on to SONET STS signals. An important aspect for transparency is to preserve the original timing information of the subscriber signal. Accordingly, it would be highly desirable to provide improved methods of synchronization and desynchronization that redress the deficiencies of the prior art as described above.